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Mobile Raman spectroscopy in astrobiology research Peter Vandenabeele1 and Jan Jehlička2 1 Department of archaeology, Ghent University,
rsta.royalsocietypublishing.org
Review Cite this article: Vandenabeele P, Jehlička J. 2014 Mobile Raman spectroscopy in astrobiology research. Phil. Trans. R. Soc. A 372: 20140202. http://dx.doi.org/10.1098/rsta.2014.0202
One contribution of 14 to a Theme Issue ‘Raman spectroscopy meets extremophiles on Earth and Mars: studies for successful search of life’.
Subject Areas: astrobiology, analytical chemistry, spectroscopy, biogeochemistry Keywords: astrobiology, Raman spectroscopy, mobile Raman instrumentation, fieldwork Author for correspondence: Peter Vandenabeele e-mail: [email protected]
Sint-Pietersnieuwstraat 35, 9000 Ghent, Belgium 2 Institute of Geochemistry, Mineralogy and Mineral Resources, Charles University, Prague, Czech Republic Raman spectroscopy has proved to be a very useful technique in astrobiology research. Especially, working with mobile instrumentation during fieldwork can provide useful experiences in this field. In this work, we provide an overview of some important aspects of this research and, apart from defining different types of mobile Raman spectrometers, we highlight different reasons for this research. These include gathering experience and testing of mobile instruments, the selection of target molecules and to develop optimal data processing techniques for the identification of the spectra. We also identify the analytical techniques that it would be most appropriate to combine with Raman spectroscopy to maximize the obtained information and the synergy that exists with Raman spectroscopy research in other research areas, such as archaeometry and forensics.
1. Introduction For several years, Raman spectroscopy has proved to be useful in astrobiology research [1–3]. Indeed, this technique has several properties that are very useful in this challenging research field. The method is relatively fast and by providing molecular spectroscopic information, the approach is useful for characterizing both mineral phases and biological materials. The major advantage for planetary research and exobiology is that inorganic as well as organic objects can be detected. Since its discovery in 1928, Raman spectroscopy has evolved from a complex spectroscopic technique towards an approach that is easily accessible in many research laboratories [4]. This evolution was made possible by many technical improvements (e.g. the introduction of the laser, charge-coupled device (CCD)
2014 The Author(s) Published by the Royal Society. All rights reserved.
Downloaded from http://rsta.royalsocietypublishing.org/ on July 21, 2015
When discussing field applications of Raman spectroscopy, it is good to distinguish between different types of mobile Raman instruments [4,5]. In general, we speak about mobile Raman instrumentation, when they are designed for mobile use. Indeed, almost all laboratory Raman spectrometers are ‘transportable’: they can be transported to different locations and after some set-up (e.g. alignment of laser inside the instrument), they are operational. The design for mobility extends to different degrees. Portable instruments are mobile spectrometers that typically can be brought to the site by a single person. It is typically battery operated and is often a fibre-optics-based instrument: in that case, a probehead is positioned in front of the object that is analysed. The spectrometer is usually operated by an internal or external laptop computer. These instruments typically weigh around 10 kg and the spectrometer is mounted in a suitcase that fits in the limits for carry-on luggage allowance. Handheld Raman instruments are typically rather small portable Raman spectrometers, which can be operated while being held in one hand. The spectrometer (and often also the spectral identification through comparison with a database) is typically performed on a small Personal digital assistant computer, equipped with a touchscreen, which is built-in into the spectrometer. This instrument type typically weighs up to few kilograms. Palm instruments are even smaller and fit into the palm of the hand of the operator, and the spectrometer has very small dimensions. As a consequence, the spectral resolution is relatively low and the size of the spectral window is limited. The weight of this instrument is typically below 1 kg. Another series of instruments, which can be considered, are Mars prototypes of various constructions that have been tested in the frame of preparative studies under laboratory environments [6].
3. Mobile Raman spectroscopy in astrobiology research Raman spectroscopy was first proposed for planetary applications by Wang et al. [7] and Wdowiak et al. [8]. Subsequently, new Raman systems have been tested with the goal of analysing minerals [9]. Raman spectroscopy has been repeatedly suggested as a powerful technique to detect mineralogical and biological markers for Mars lander missions [10–13]. The option to remotely identify mineral and organic targets was proposed by Sharma et al. [14]. The stand-off mode of working was later developed and improved [12,15], and recently stand-off UV Raman spectroscopy has been demonstrated to have good potential for targeting organic and inorganic objects for future Mars research [16]. Another mode of operation of this technique would be a surface investigation using a miniature Raman spectrometer connected to a robotic arm with a short focal distance [17]. The initial proposal for the ExoMars lander mission included another approach, consisting of analyses of powdered samples originating from drilling operations by a combination of Laser-induced breakdown spectroscopy (LIBS) methods and Raman spectroscopy [18]; however, this concept was de-scoped, and the LIBS instrument was dropped. During space missions, the payload has to be minimized [19,20]. Several prototypes of the Raman instruments to later be included in the future Mars missions are nowadays tested in the frame of NASA and ESA projects. Space agencies have been evaluating the advantages and limitations of using excitation lasers of different wavelengths, with the most
.........................................................
2. Mobile Raman instrumentation
2
rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 372: 20140202
detectors, microscope optics, etc.) leading to compact—yet sensitive—Raman spectrometers. Mobile Raman instrumentation is designed for experiments outside the laboratory. Different degrees of mobility should be distinguished: weight and size constraints and the availability of batteries (as opposed to an external power source) are major issues. Moreover, the dimensions of a dispersive spectrometer have implications on the achievable spectral resolution.
Downloaded from http://rsta.royalsocietypublishing.org/ on July 21, 2015
In this context, experiments in the so-called ‘Mars-analogue sites on earth’ are crucial [23]. Often the field measurements are completed with investigations of collected specimens in the laboratory. Instruments are subjected to extreme conditions and the obtained spectral quality is evaluated. These conditions may include high altitude [24,25] and extreme cold [26,27], arid conditions [28,29], high saline content [30], etc. Experience is gained in recording Raman spectra under different field-conditions and solutions for arising problems or interferences (e.g. ambient light) are identified.
(b) Selection of target molecules Different groups of materials can be examined by using Raman spectroscopy. Analysis of the mineral composition of the rocks is a straightforward target, and several experiments have been performed in this field [31–34]. However, in order to understand better the genesis conditions of these minerals, the examination of micro-inclusions inside the minerals should be a next target. A third group of target materials for astrobiology research are the so-called biomarker molecules that are specific markers for the presence of extinct or extant forms of life [35,36]. It is obvious that the target molecules also influence the different instrumental options that have to be taken (e.g. selection of laser wavelength) and hands-on experience on the Earth provides insight in what works and what might cause failure due to fluorescence or self-absorption.
(c) Data processing and identification As (due to energy and timing issues) there are constraints in the communication between the rover and the control centre on the Earth, data processing should be optimized, so that the scientific information is reduced to a small datastream. The transfer of useless information should be minimized and the use of redundant information should be kept minimal, provided that this does not jeopardize read-out of the datastream. On the one hand, one should be sure that
.........................................................
(a) Testing of the instrumental performance and miniaturization, based on field analysis on the Earth
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rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 372: 20140202
recent decision taken being to send a miniature Raman instrument equipped with a 532 nm laser [12]. A laboratory model of the ExoMars Raman Laser Spectrometer (RLS) device was developed at the Associated Unit University of Valladolid-CSICCenter of Astrobiology (UVaCAB) [12,21,22]. The Raman prototype (532 nm) has an XYZ positioning system with 2.5 µm precision and imaging, autofocus and spectral acquisition capabilities. The analytical strategy of the RLS instrument is the acquisition of spectra along a linear series of points. At each point, the optical head is focused on the sample, the acquisition parameters are automatically calculated and the spectrum acquired. The laser spot size is of 50 µm diameter yielding an irradiance level between 0.6 and 1.2 kW cm−2 , with a 6–7 cm−1 spectral resolution. The UK Bread-Board Raman spectrometer system was developed to permit to characterize and optimize the performance of the ExoMars RLS detector assembly [6]. The prototype system consists of a continuous wave diode laser (100 mW at 532 nm) a detector system incorporating a thermoelectrically cooled CCD and drive/acquisition electronics and a transmission grating with two spectral orders (the spectrometer units provide sensitivity across the 200–3500 cm−1 wavenumber range with a spectral resolution of less than 10 cm−1 . Data are collected using a range of different integration times; best results were typically obtained for integration times in the range 10–60 s, laser power settings being adjusted using filters. Recent research papers on the use of mobile Raman instrumentation in astrobiology have different research goals. These include, among others, the testing and optimization of mobile instrumentation in the field, the selection of target molecules, the optimization of the data processing and spectral identification and the combination with other analytical techniques. In this approach, a strong parallelism can be seen with in situ Raman analysis in other research fields.
Downloaded from http://rsta.royalsocietypublishing.org/ on July 21, 2015
Another aspect of field analysis is the combination with other analytical techniques. During fieldwork, it is obvious for a geo-scientist to combine the results from spectroscopy with his/her observations of texture, colour, shape, environment, etc. However, when considering the analysis of specimens on the surface of Mars, all tactile experiences are lost and one should rely entirely on the spectroscopic results. Often, it can be of use to combine different spectroscopic techniques that provide complementary information. Many combinations have been considered—typically combining elemental with molecular information. In this context, the combination Raman/LIBS or the combination Raman/XRF is often encountered. The first approach has the advantage that a device can be developed that uses the same laser (eventually adapted with nonlinear optical components to double the laser frequency and to modulate laser power) for both types of experiments. The latter approach (Raman/XRF) has the advantage that for both techniques portable instruments are commercially available on the market and thus, they can easily be tested during fieldwork. These experiments help in optimizing the approach, seeing the pitfalls during the practical work and also in improving the data processing.
(e) Parallelism with other research areas It can be remarked that there is a strong parallelism between the challenges that are encountered in applying Raman spectroscopy in astrobiology research and some aspects of using mobile Raman instrumentation for other applications, like gemmology, archaeometry, art analysis or forensics applications. In these research fields, several aspects of the research are in common, such as the need for stable positioning equipment, avoiding interferences from ambient factors, or the difficulty to bring the instruments on site. In archaeometry and art analysis, one of the main questions is often to maximize the information while minimizing the sample amount, or when performing direct analysis—a similar challenge is found when designing a mobile spectrometer for remote investigations in space.
4. Conclusion The optimization and miniaturization of the technologies associated with Raman spectroscopy have made this technique suitable for application in the field and for astrobiology research. Applying these instruments during fieldwork provides useful information for future missions to Mars. Different categories of Raman spectrometers should be distinguished, such as transportable, mobile, portable, handheld and palm instruments. Mobile Raman spectroscopy is used in astrobiology analogue research for a wide range of different reasons. One of the main reasons is gathering experience with this type of instrumentation, in order to optimize the approach for a possible future mission to Mars. Moreover, a selection has to be made of possible target molecules to study. Among these, biomarker molecules, which are indicative for extinct or extant life, have a high priority. Owing to technical constraints in the data communication, data processing should be automated and robust identification algorithms are important. Then, the results should also be combined with other techniques, like LIBS or XRF, and a parallelism with other research fields can be observed.
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(d) Combination with other techniques
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no crucial information is lost, while on the other hand, it is of the utmost importance that no ambiguous data are introduced. Therefore, it is necessary to perform automated calibration and data processing [37] on site and the spectrometer system should be able to differentiate between useless spectra (e.g. only fluorescence background information) and Raman data. Hence, it is necessary to develop and test these algorithms, as well as to develop reliable databases for identification of the molecules. An important issue in this context is the determination of the limit of identification and limit of detection of these materials, in specific matrices [38].
Downloaded from http://rsta.royalsocietypublishing.org/ on July 21, 2015
1. Wynn-Williams DD, Newton EM, Edwards HGM. 2001 The role of habitat structure for biomolecule integrity and microbial survival under extreme environmental stress in Antarctica (and Mars?): ecology and technology. In Exo-/astro-biology, vol. 496 (eds P Ehrenfreund, O Angerer, B Battrick), pp. 225–237. Noordwijk, The Netherlands: ESA special publications. 2. Ellery A, Wynn-Williams D. 2003 Why Raman Spectroscopy on mars? A case of the right tool for the right job. Astrobiology 3, 565–579. (doi:10.1089/153110703322610654) 3. Popp J, Schmitt M. 2004 Raman spectroscopy breaking terrestrial barriers! J. Raman Spectrosc. 35, 429–432. (doi:10.1002/jrs.1198) 4. Vandenabeele P, Edwards HGM, Jehlicka J. 2014 The role of mobile instrumentation in novel applications of Raman spectroscopy: archaeometry, geosciences, and forensics. Chem. Soc. Rev. 43, 2628–2649. (doi:10.1039/c3cs60263j) 5. Lauwers D, Hutado AG, Tanevska V, Moens L, Bersani D, Vandenabeele P. 2014 Characterisation of a portable Raman spectrometer for in situ analysis of art objects. Spectrochim. Acta Part A 118, 294–301. (doi:10.1016/j.saa.2013.08.088) 6. Edwards HGM, Hutchinson I, Ingley R. 2012 The ExoMars Raman spectrometer and the identification of biogeological spectroscopic signatures using a flight-like prototype. Anal. Bioanal. Chem. 404, 1723–1731. (doi:10.1007/s00216-012-6285-z) 7. Wang A, Jolliff BL, Haskin LA. 1995 Raman spectroscopy as a method for mineral identification on lunar robotic exploration missions. J. Geophys. Res. 100, 21 189–21 199. (doi:10.1029/95JE02133) 8. Wdowiak T, Agresti D, Mirov S. 1995 A laser Raman spectrometer system suitable for incorporation into lander spacecraft. Lunar Planet. Inst. Sci. Conf. Abs. 23, 1473. 9. Haskin LA, Wang A, Rockow KM, Jolliff BL, Korotev RL, Viskupic KM. 1997 Raman spectroscopy for mineral identification and quantification for in situ planetary surface analysis: a point count method. J. Geophys. Res. Planets 102, 19 293–19 306. (doi:10.1029/97JE01694) 10. Edwards HGM, Moody CD, Newton EM, Villar SEJ, Russell MJ. 2005 Raman spectroscopic analysis of cyanobacterial colonization of hydromagnesite, a putative Martian extremophile. Icarus 2, 372–381. (doi:10.1016/j.icarus.2004.12.006) 11. Villar SEJ, Edwards HGM. 2006 Raman spectroscopy in astrobiology. Anal. Bioanalyt. Chem. 384, 100–113. (doi:10.1007/s00216-005-0029-2) 12. Rull F, Vegas A, Sansano A, Sobron P. 2011 Analysis of arctic ices by remote Raman spectroscopy. Spectrochim. Acta Part A 80, 148–155. (doi:10.1016/j.saa.2011.04.007) 13. Edwards HGM, Hutchinson IB, Ingley R, Parnell J, Vitek P, Jehlicka J. 2013 Raman spectroscopic analysis of geological and biogeological specimens of relevance to the ExoMars mission. Astrobiology 13, 543–549. (doi:10.1089/ast.2012.0872) 14. Sharma SK, Misra AK, Lucey PG, Angel SM, McKay CP. 2006 Remote pulsed Raman spectroscopy of inorganic and organic materials to a radial distance of 100 meters. Appl. Spectrosc. 60, 871–876. (doi:10.1366/000370206778062110) 15. Misra AK, Sharma SK, Acosta TE, Porter JN, Bates DE. 2012 Single-pulse standoff Raman detection of chemicals from 120 m distance during daytime. Appl. Spectrosc. 66, 1279–1285. (doi:10.1366/12-06617) 16. Skulinova M et al. 2014 Time-resolved stand-off UV-Raman spectroscopy for planetary exploration. Planet. Space Sci. 92, 88–100. (doi:10.1016/j.pss.2014.01.010) 17. Wang A, Haskin LA, Cortez E. 1998 Prototype Raman spectroscopic sensor for in situ mineral characterization on planetary surfaces. Appl. Spectrosc. 52, 477–487. (doi:10.1366/ 0003702981943842) 18. Courrèges-Lacoste GB, Ahlers B, Perez FR. 2007 Combined Raman spectrometer/laserinduced spectrometer for the next ESA mission to breakdown Mars. Spectrochim. Acta Part A 68, 1023–1028. (doi:10.1016/j.saa.2007.03.026) 19. Hutchinson IB, Ingley R, Edwards HGM, Harris L, McHugh M, Malherbe C, Parnell J. 2014 Raman spectroscopy on Mars: identification of geological and bio-geological signatures
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References
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Funding statement. P.V. thanks the Ghent University Special Research Fund (BOF-UGent) for the financial support through the ongoing GOA project. This work was partly funded by a grant of the Grant Agency of the Czech Republic no. P210/10/0467 and by institutional support no. MSM0021620855 from the Ministry of Education of the Czech Republic.
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Mobile Raman spectroscopy in astrobiology research Peter Vandenabeele1 and Jan Jehlička2 1 Department of archaeology, Ghent University,
rsta.royalsocietypublishing.org
Review Cite this article: Vandenabeele P, Jehlička J. 2014 Mobile Raman spectroscopy in astrobiology research. Phil. Trans. R. Soc. A 372: 20140202. http://dx.doi.org/10.1098/rsta.2014.0202
One contribution of 14 to a Theme Issue ‘Raman spectroscopy meets extremophiles on Earth and Mars: studies for successful search of life’.
Subject Areas: astrobiology, analytical chemistry, spectroscopy, biogeochemistry Keywords: astrobiology, Raman spectroscopy, mobile Raman instrumentation, fieldwork Author for correspondence: Peter Vandenabeele e-mail: [email protected]
Sint-Pietersnieuwstraat 35, 9000 Ghent, Belgium 2 Institute of Geochemistry, Mineralogy and Mineral Resources, Charles University, Prague, Czech Republic Raman spectroscopy has proved to be a very useful technique in astrobiology research. Especially, working with mobile instrumentation during fieldwork can provide useful experiences in this field. In this work, we provide an overview of some important aspects of this research and, apart from defining different types of mobile Raman spectrometers, we highlight different reasons for this research. These include gathering experience and testing of mobile instruments, the selection of target molecules and to develop optimal data processing techniques for the identification of the spectra. We also identify the analytical techniques that it would be most appropriate to combine with Raman spectroscopy to maximize the obtained information and the synergy that exists with Raman spectroscopy research in other research areas, such as archaeometry and forensics.
1. Introduction For several years, Raman spectroscopy has proved to be useful in astrobiology research [1–3]. Indeed, this technique has several properties that are very useful in this challenging research field. The method is relatively fast and by providing molecular spectroscopic information, the approach is useful for characterizing both mineral phases and biological materials. The major advantage for planetary research and exobiology is that inorganic as well as organic objects can be detected. Since its discovery in 1928, Raman spectroscopy has evolved from a complex spectroscopic technique towards an approach that is easily accessible in many research laboratories [4]. This evolution was made possible by many technical improvements (e.g. the introduction of the laser, charge-coupled device (CCD)
2014 The Author(s) Published by the Royal Society. All rights reserved.
Downloaded from http://rsta.royalsocietypublishing.org/ on July 21, 2015
When discussing field applications of Raman spectroscopy, it is good to distinguish between different types of mobile Raman instruments [4,5]. In general, we speak about mobile Raman instrumentation, when they are designed for mobile use. Indeed, almost all laboratory Raman spectrometers are ‘transportable’: they can be transported to different locations and after some set-up (e.g. alignment of laser inside the instrument), they are operational. The design for mobility extends to different degrees. Portable instruments are mobile spectrometers that typically can be brought to the site by a single person. It is typically battery operated and is often a fibre-optics-based instrument: in that case, a probehead is positioned in front of the object that is analysed. The spectrometer is usually operated by an internal or external laptop computer. These instruments typically weigh around 10 kg and the spectrometer is mounted in a suitcase that fits in the limits for carry-on luggage allowance. Handheld Raman instruments are typically rather small portable Raman spectrometers, which can be operated while being held in one hand. The spectrometer (and often also the spectral identification through comparison with a database) is typically performed on a small Personal digital assistant computer, equipped with a touchscreen, which is built-in into the spectrometer. This instrument type typically weighs up to few kilograms. Palm instruments are even smaller and fit into the palm of the hand of the operator, and the spectrometer has very small dimensions. As a consequence, the spectral resolution is relatively low and the size of the spectral window is limited. The weight of this instrument is typically below 1 kg. Another series of instruments, which can be considered, are Mars prototypes of various constructions that have been tested in the frame of preparative studies under laboratory environments [6].
3. Mobile Raman spectroscopy in astrobiology research Raman spectroscopy was first proposed for planetary applications by Wang et al. [7] and Wdowiak et al. [8]. Subsequently, new Raman systems have been tested with the goal of analysing minerals [9]. Raman spectroscopy has been repeatedly suggested as a powerful technique to detect mineralogical and biological markers for Mars lander missions [10–13]. The option to remotely identify mineral and organic targets was proposed by Sharma et al. [14]. The stand-off mode of working was later developed and improved [12,15], and recently stand-off UV Raman spectroscopy has been demonstrated to have good potential for targeting organic and inorganic objects for future Mars research [16]. Another mode of operation of this technique would be a surface investigation using a miniature Raman spectrometer connected to a robotic arm with a short focal distance [17]. The initial proposal for the ExoMars lander mission included another approach, consisting of analyses of powdered samples originating from drilling operations by a combination of Laser-induced breakdown spectroscopy (LIBS) methods and Raman spectroscopy [18]; however, this concept was de-scoped, and the LIBS instrument was dropped. During space missions, the payload has to be minimized [19,20]. Several prototypes of the Raman instruments to later be included in the future Mars missions are nowadays tested in the frame of NASA and ESA projects. Space agencies have been evaluating the advantages and limitations of using excitation lasers of different wavelengths, with the most
.........................................................
2. Mobile Raman instrumentation
2
rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 372: 20140202
detectors, microscope optics, etc.) leading to compact—yet sensitive—Raman spectrometers. Mobile Raman instrumentation is designed for experiments outside the laboratory. Different degrees of mobility should be distinguished: weight and size constraints and the availability of batteries (as opposed to an external power source) are major issues. Moreover, the dimensions of a dispersive spectrometer have implications on the achievable spectral resolution.
Downloaded from http://rsta.royalsocietypublishing.org/ on July 21, 2015
In this context, experiments in the so-called ‘Mars-analogue sites on earth’ are crucial [23]. Often the field measurements are completed with investigations of collected specimens in the laboratory. Instruments are subjected to extreme conditions and the obtained spectral quality is evaluated. These conditions may include high altitude [24,25] and extreme cold [26,27], arid conditions [28,29], high saline content [30], etc. Experience is gained in recording Raman spectra under different field-conditions and solutions for arising problems or interferences (e.g. ambient light) are identified.
(b) Selection of target molecules Different groups of materials can be examined by using Raman spectroscopy. Analysis of the mineral composition of the rocks is a straightforward target, and several experiments have been performed in this field [31–34]. However, in order to understand better the genesis conditions of these minerals, the examination of micro-inclusions inside the minerals should be a next target. A third group of target materials for astrobiology research are the so-called biomarker molecules that are specific markers for the presence of extinct or extant forms of life [35,36]. It is obvious that the target molecules also influence the different instrumental options that have to be taken (e.g. selection of laser wavelength) and hands-on experience on the Earth provides insight in what works and what might cause failure due to fluorescence or self-absorption.
(c) Data processing and identification As (due to energy and timing issues) there are constraints in the communication between the rover and the control centre on the Earth, data processing should be optimized, so that the scientific information is reduced to a small datastream. The transfer of useless information should be minimized and the use of redundant information should be kept minimal, provided that this does not jeopardize read-out of the datastream. On the one hand, one should be sure that
.........................................................
(a) Testing of the instrumental performance and miniaturization, based on field analysis on the Earth
3
rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 372: 20140202
recent decision taken being to send a miniature Raman instrument equipped with a 532 nm laser [12]. A laboratory model of the ExoMars Raman Laser Spectrometer (RLS) device was developed at the Associated Unit University of Valladolid-CSICCenter of Astrobiology (UVaCAB) [12,21,22]. The Raman prototype (532 nm) has an XYZ positioning system with 2.5 µm precision and imaging, autofocus and spectral acquisition capabilities. The analytical strategy of the RLS instrument is the acquisition of spectra along a linear series of points. At each point, the optical head is focused on the sample, the acquisition parameters are automatically calculated and the spectrum acquired. The laser spot size is of 50 µm diameter yielding an irradiance level between 0.6 and 1.2 kW cm−2 , with a 6–7 cm−1 spectral resolution. The UK Bread-Board Raman spectrometer system was developed to permit to characterize and optimize the performance of the ExoMars RLS detector assembly [6]. The prototype system consists of a continuous wave diode laser (100 mW at 532 nm) a detector system incorporating a thermoelectrically cooled CCD and drive/acquisition electronics and a transmission grating with two spectral orders (the spectrometer units provide sensitivity across the 200–3500 cm−1 wavenumber range with a spectral resolution of less than 10 cm−1 . Data are collected using a range of different integration times; best results were typically obtained for integration times in the range 10–60 s, laser power settings being adjusted using filters. Recent research papers on the use of mobile Raman instrumentation in astrobiology have different research goals. These include, among others, the testing and optimization of mobile instrumentation in the field, the selection of target molecules, the optimization of the data processing and spectral identification and the combination with other analytical techniques. In this approach, a strong parallelism can be seen with in situ Raman analysis in other research fields.
Downloaded from http://rsta.royalsocietypublishing.org/ on July 21, 2015
Another aspect of field analysis is the combination with other analytical techniques. During fieldwork, it is obvious for a geo-scientist to combine the results from spectroscopy with his/her observations of texture, colour, shape, environment, etc. However, when considering the analysis of specimens on the surface of Mars, all tactile experiences are lost and one should rely entirely on the spectroscopic results. Often, it can be of use to combine different spectroscopic techniques that provide complementary information. Many combinations have been considered—typically combining elemental with molecular information. In this context, the combination Raman/LIBS or the combination Raman/XRF is often encountered. The first approach has the advantage that a device can be developed that uses the same laser (eventually adapted with nonlinear optical components to double the laser frequency and to modulate laser power) for both types of experiments. The latter approach (Raman/XRF) has the advantage that for both techniques portable instruments are commercially available on the market and thus, they can easily be tested during fieldwork. These experiments help in optimizing the approach, seeing the pitfalls during the practical work and also in improving the data processing.
(e) Parallelism with other research areas It can be remarked that there is a strong parallelism between the challenges that are encountered in applying Raman spectroscopy in astrobiology research and some aspects of using mobile Raman instrumentation for other applications, like gemmology, archaeometry, art analysis or forensics applications. In these research fields, several aspects of the research are in common, such as the need for stable positioning equipment, avoiding interferences from ambient factors, or the difficulty to bring the instruments on site. In archaeometry and art analysis, one of the main questions is often to maximize the information while minimizing the sample amount, or when performing direct analysis—a similar challenge is found when designing a mobile spectrometer for remote investigations in space.
4. Conclusion The optimization and miniaturization of the technologies associated with Raman spectroscopy have made this technique suitable for application in the field and for astrobiology research. Applying these instruments during fieldwork provides useful information for future missions to Mars. Different categories of Raman spectrometers should be distinguished, such as transportable, mobile, portable, handheld and palm instruments. Mobile Raman spectroscopy is used in astrobiology analogue research for a wide range of different reasons. One of the main reasons is gathering experience with this type of instrumentation, in order to optimize the approach for a possible future mission to Mars. Moreover, a selection has to be made of possible target molecules to study. Among these, biomarker molecules, which are indicative for extinct or extant life, have a high priority. Owing to technical constraints in the data communication, data processing should be automated and robust identification algorithms are important. Then, the results should also be combined with other techniques, like LIBS or XRF, and a parallelism with other research fields can be observed.
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(d) Combination with other techniques
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no crucial information is lost, while on the other hand, it is of the utmost importance that no ambiguous data are introduced. Therefore, it is necessary to perform automated calibration and data processing [37] on site and the spectrometer system should be able to differentiate between useless spectra (e.g. only fluorescence background information) and Raman data. Hence, it is necessary to develop and test these algorithms, as well as to develop reliable databases for identification of the molecules. An important issue in this context is the determination of the limit of identification and limit of detection of these materials, in specific matrices [38].
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Funding statement. P.V. thanks the Ghent University Special Research Fund (BOF-UGent) for the financial support through the ongoing GOA project. This work was partly funded by a grant of the Grant Agency of the Czech Republic no. P210/10/0467 and by institutional support no. MSM0021620855 from the Ministry of Education of the Czech Republic.
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